Many low power isolated supplies used in e-meters, industrial controls, motor drives, and medical applications, for example, applications use an open loop power supply to power circuits on the secondary side. The outputs of the open loop power supplies are generally not regulated and do not have soft start to minimize inrush currents. These issues can be circumvented by regulating the primary-side of the supply, however. Prior art FIG. 1 shows a simplified schematic of such isolated supply 100. The power supply 100 is a variation of a Flyback converter and consists of a half bridge power stage S1 and S2, transformer T1, primary side capacitor Cr, diode D1, and output capacitor Co.
Although the supply 100 shown in FIG. 1 provides better regulation compared to open-loop systems, the regulation can improve by sensing the output changes at the secondary-side using an optocoupler 210 as shown in Prior art FIG. 2 or using a linear regulator (not shown) at the secondary-side to regulate the output even further as discussed below.
In isolated switching power supplies, optocouplers 210 are widely used to provide isolation in the feedback loop. One of the disadvantages of using optocouplers is its low bandwidth, because the bandwidth is reduced by introducing an extra pole in the control loop gain of the supply. Another disadvantage of using optocouplers is the large unit-to-unit variation in the current transfer ratio (CTR). The CTR or the coupling efficiency is defined as the ratio of opto-isolator transistor collector current to the diode current. High variation in CTR imposes constraints on control loop design for power supplies and causes the compensation of the loop to be more difficult. In addition, optocouplers are not as reliable at high temperatures, and therefore not preferred for space, military, and some industrial applications.
Instead of optocouplers, a linear regulator can regulate the output down to a desired output voltage level. This strategy helps avoiding bandwidth, process variation, and reliability concerns associated with optocouplers. Although the output can be tightly regulated with a linear regulator, it also results in efficiency loss mainly due to power dissipation across the pass element of the regulator.